A polymer electrolyte fuel cell, for example, is known as one type of fuel cell which generates electrical power utilizing an electrochemical reaction between hydrogen and oxygen. The polymer electrolyte fuel cell includes a stack which is constituted of a plurality of stacked cells. The cells constituting the stack each include an anode (fuel electrode) and a cathode (air electrode), and a solid polymer electrolyte membrane having a sulfonic acid group as an ion exchange group is interposed between each anode and cathode.
A fuel gas (hydrogen-enriched reformed hydrogen obtained by reforming hydrogen gas or hydrocarbon) is supplied to the anode, while an oxidant gas containing oxygen (for example, air) is supplied to the cathode as an oxidant. Upon the supply of the fuel gas to the anode, hydrogen contained in the fuel gas reacts with a catalyst in a catalyst layer which constitutes the anode, thereby producing hydrogen ions. The hydrogen ions produced thereby pass through the solid polymer electrolyte membrane and electrically react with oxygen in the cathode. Electrical power is thus generated through the electrochemical reaction.
As an example of such a fuel cell system, the invention disclosed in patent document 1 described below is suggested. The fuel cell system disclosed in patent document 1 described below suppresses a variation in the cell voltage of the fuel cell body while eliminating the waste of source gas and raw material supply power. Specifically, when a variation occurs in the voltage of a power-generating cell that constitutes the fuel cell body, a determination is made as to whether or not the cause is due to a source gas distribution inside the fuel cell body or a cell degradation, and a source gas stoichiometry ratio is set in accordance with the determined result.
As an example of such a fuel cell system, the invention disclosed in patent document 2 described below is suggested. The fuel cell system disclosed in patent document 2 described below does not prompt a user to inspect or exchange when there is a recoverable decrease in the current-voltage characteristic of a fuel cell, but is capable of prompting the user to inspect or exchange when the non-recoverable amount of decrease in the current-voltage characteristic of the fuel cell reaches a predetermined value. This fuel cell system estimates the current-voltage characteristic of the fuel cell body to judge the degradation of the fuel cell body. Specifically, the amount of decrease in the current-voltage characteristic from the pre-stored initial value V1 of the current-voltage characteristic to the present voltage value V3 is referred to as a first decreased amount, and the recoverable amount of decrease in the current-voltage characteristic when the operation is stopped is referred to as a second decreased amount. A third decreased amount obtained by subtracting the second decreased amount from the first decreased amount is a non-recoverable amount of decrease, and when the third decreased amount reaches the predetermined value, the user is prompted to inspect or exchange the fuel cell body.
As an example of such a fuel cell system, the invention disclosed in patent document 3 described below is suggested. The fuel cell system disclosed in patent document 3 described below stably extracts output without causing a hunting of an output limitation. Specifically, when an output voltage of the fuel cell falls below a voltage lower limit threshold value at which the fuel cell is capable of stably generating power, the current to be extracted from the fuel cell is limited, and when the output voltage rises and reaches the voltage lower limit threshold or higher, the limitation for the current to be extracted from the fuel cell is released, and further, the upper limit for the amount of increase in the current when the limitation for the current is released is computed.